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Sorry should have been clearer. The pressure when on. Always 1.2 at cold but now 2.3 at 51C and higher the hotter we go....

So I tried to burn the gas - it blew the match out so not Hydrogen. I cracked open the manual vent and got soaked - so no air there. I have noticed the pressure us rising higher by the day - expansion vessel issues?

So I tried to burn the gas - it blew the match out so not Hydrogen. I cracked open the manual vent and got soaked - so no air there. I have noticed the pressure us rising higher by the day - expansion vessel issues?

Yup. Vent when off and cold. Its a vaillant 637 system boiler. Its about 3m away from the tank and valves on the same floor. The manual vent thing, new one on me, should I just crack that open and see if there is air in there?

Its the pipe to nowhere. On the flow at the split between DHW and CH.

Terrywookfit, there is an aav in the boiler but not on the pipework. Interestingly there is a 15mm vertical deadleg on the flow at the point the CH and DHW valves are. In some pictures I've seen this is common but tends to have an aav on it. Mine does not. Should it?

Hope someone can help. I regularly get significant air in my CH that I can bleed out only for it to return a week or so later. The obvious answer is there is a leak somewhere but the pressure in the system does not drop until such time as I bleed the rads. If I ignore it / leave it for long...